For example, in the processes of manufacturing a NAND flush memory or a MRAM (magneto-resistive random-access memory), processes of forming insulator films such as aluminum oxide films, magnesium oxide films, and the like are carried out. Sputtering apparatus is used in order to form insulator films with good productivity. In this kind of sputtering apparatus, inside a vacuum chamber that can be evacuated, there is detachably assembled a target assembly which is made by integrating a target that is appropriately selected depending on the composition of a thin film to be formed, and a backing plate for cooling the target at the time of film forming by sputtering.
This kind of target assembly is known, e.g., in Patent Document 1. According to this arrangement, the backing plate is formed of metal such as copper with good thermal conduction, and has a projected portion projected outward beyond an outer peripheral end of the target. By taking advantage of this projected portion, it is so arranged that the target assembly can be fixed to a predetermined position of the sputtering apparatus. Further, after having assembled the target assembly into the sputtering apparatus, an annular shield plate is generally disposed opposite to the projected portion, for the purpose of stabilizing the discharging, etc.
By the way, in a state in which the target assembly and the shield plate have been assembled into the sputtering apparatus, there is a clearance between the side surface of the target and the shield plate. When plasma is caused to be generated in the vacuum chamber at the time of film forming, the electrons in the plasma will sometimes be electrostatically charged, through the above-mentioned clearance, in the projected portion that is made of metal. When the projected portion gets charged with electrons, since the target is made of an insulating material, abnormal discharging occurs due to the potential difference between the side surface of the target and the projected portion. Due to this phenomenon, there is a case in which the bonding material gets seeped to the outside. If film forming is performed in this kind of state, there will occur so-called contamination in which metal gets mixed into the insulation film that is formed on the surface of a substrate. This will prevent good film formation.
In order to prevent the bonding material from seeping to the outside, the applicant of this patent application has once proposed a cathode assembly in a prior patent application (Japanese Patent Application No. 2014-163095). What the applicant proposed in the patent application was a cathode assembly: in which the bonding portion of the backing plate to which a target is bonded, is protruded relative to the projected portion; the surfaces are roughened from the projected portion to the side surface of the bonded portion; and the film of insulating material is formed to extend from the projected portion to the side surface of the target. In case the life time of the backing plate is longer than that of the target, the backing plate is reused. In other words, once a predetermined number of processed products has reached, the target assembly is taken out of the sputtering apparatus, the bonding material is removed to separate the used target from the backing plate, and an unused target is bonded to the backing plate by means of a bonding material. The target assembly obtained in this manner will be assembled in the sputtering apparatus.
In the above-mentioned cathode assembly of the above-mentioned prior patent application, before the bonding material is removed, the insulator film made of an insulating material must be removed. But the removal of this insulator film is very difficult. Further, after bonding the unused target to the backing plate, an insulator film must be formed once again. It takes much time in reusing the backing plate.